Leukocyte antigen CD37 (“CD37”), also known as GP52-40, tetraspanin-26, or TSPAN26, is a transmembrane protein of the tetraspanin superfamily (Maecker et al., 1997 FASEB J. 11:428-442). It is a heavily glycosylated protein with four transmembrane domains that is expressed on B cells during the pre-B to peripheral mature B-cell stages, but is reportedly absent on terminal differentiation to plasma cells. (Link et al., 1987, J Pathol. 152:12-21). The CD37 antigen is only weakly expressed on T-cells, myeloid cells, and granulocytes (Schwartz-Albiez et al. 1988, J. Immunol., 140(3)905-914). However, CD37 is also expressed on malignant B-cells such as those founding non-Hodgkin's lymphoma (NHL) and chronic lymphoid leukemia (CLL) (Moore et al. 1986, J Immunol. 137(9):3013-8).
While the exact physiological role of CD37 is unclear, studies in CD37-deficient mice suggest an immunoregulatory function. Although mice deficient in CD37 expression have normal development (Knobeloch et al. 2000, Mol Cell Biol., 20(15):5363-9), in the C57/B16 background, CD37−/− T cells are hyper-proliferative (van Spriel et al., J Immunol. 172, 2953 (2004)), CD37−/− dendritic cells (DC) exhibit an increased antigen presentation (Sheng et al., Eur J Immunol. 39, 50 (2009)), and CD37−/− macrophages show increased dectin-1-induced IL-6 production (Meyer-Wentrup et al., J Immunol. 178, 154 (2007)). CD37-deficient C57/B16 mice also contain significantly higher level of IgA than the wild-type mice (van Spriel et al., PLoS Pathol. 5, e1000338 (2009) and Rops et al., Am J Pathol. 176, 2188 (2010)). All of these results suggest a general regulatory role of CD37 in the immune system. Interestingly, crosslinking of CD37 antigen by antibody on human T cells inhibits T cell proliferation induced by CD3 stimulation (van Spriel et al., J Immunol. 172, 2953 (2004)).
Antibodies are emerging as a promising method to treat human diseases including autoimmune diseases. Currently, an anti-CD20 antibody called rituximab has been approved for rheumatoid arthritis (RA) treatment (Edwards J C et al. 2006, Nat Rev Immunol. 6: 119). Rituximab is used in the United States in combination with methotrexate (MTX) to reduce signs and symptoms in adult patients with moderately- to severely-active RA who have had an inadequate response to at least one TNF antagonist. Many studies address the use of rituximab in a variety of non-malignant autoimmune or inflammatory disorders, including RA, in which B-cells and autoantibodies appear to play a role in disease pathophysiology. Edwards et al., Biochem Soc. Trans. 30:824-828 (2002). Targeting of CD20 using anti-CD20 antibody has been reported to potentially relieve signs and symptoms of a number of autoimmune or inflammatory diseases including, for example, RA (Leandro et al., Ann. Rheum. Dis. 61:883-888 (2002); Edwards et al., Arthritis Rheum., 46 (Suppl. 9): S46 (2002); Stahl et al., Ann. Rheum. Dis., 62 (Suppl. 1): OP004 (2003); Emery et al., Arthritis Rheum. 48(9): S439 (2003)), lupus (Eisenberg, Arthritis. Res. Ther. 5:157-159 (2003); Leandro et al. Arthritis Rheum. 46: 2673-2677 (2002); Gorman et al., Lupus, 13: 312-316 (2004)), immune thrombocytopenic purpura (D'Arena et al., Leuk. Lymphoma 44:561-562 (2003); Stasi et al., Blood, 98: 952-957 (2001); Saleh et al., Semin. Oncol., 27 (Supp 12):99-103 (2000); Zaja et al., Haematologica, 87:189-195 (2002); Ratanatharathorn et al., Ann. Int. Med., 133:275-279 (2000)), pure red cell aplasia (Auner et al., Br. J. Haematol., 116:725-728 (2002)); autoimmune anemia (Zaja et al., supra (erratum appears in Haematologica 87:336 (2002)), cold agglutinin disease (Layios et al., Leukemia, 15:187-8 (2001); Berentsen et al., Blood, 103: 2925-2928 (2004); Berentsen et al., Br. J. Haematol., 115:79-83 (2001); Bauduer, Br. J. Haematol., 112:1083-1090 (2001); Zaja et al., Br. J. Haematol., 115:232-233 (2001)), type B syndrome of severe insulin resistance (Coll et al., N. Engl. J. Med., 350:310-311 (2004), mixed cryoglobulinermia (DeVita et al., Arthritis Rheum. 46 Suppl. 9:S206/S469 (2002)), myasthenia gravis (Zaja et al., Neurology, 55:1062-1063 (2000); Wylam et al., J. Pediatr., 143:674-677 (2003)), Wegener's granulomatosis (Specks et al., Arthritis & Rheumatism 44:2836-2840 (2001)), microscopic polyangiitis (MPA), refractory pemphigus vulgaris (Dupuy et al., Arch Dermatol., 140:91-96 (2004)), dermatomyositis (Levine, Arthritis Rheum., 46 (Suppl. 9):51299 (2002)), Sjogren's syndrome (Somer et al., Arthritis & Rheumatism, 49:394-398 (2003)), active type-II mixed cryoglobulinemia (Zaja et al., Blood, 101:3827-3834 (2003)), pemphigus vulgaris (Dupay et al., Arch. Dermatol., 140:91-95 (2004)), autoimmune neuropathy (Pestronk et al., J. Neurol. Neurosurg. Psychiatry 74:485-489 (2003)), paraneoplastic opsoclonus-myoclonus syndrome (Pranzatelli et al. Neurology 60 (Suppl. 1) PO5.128:A395 (2003)), and relapsing-remitting multiple sclerosis (RRMS). Cross et al. (abstract) “Preliminary Results from a Phase II Trial of Rituximab in MS” Eighth Annual Meeting of the Americas Committees for Research and Treatment in Multiple Sclerosis, 20-21 (2003).
In animal models, B-cell depletion using antibodies against B-cell antigens such as CD20 has been shown to inhibit or ameliorate several autoimmune diseases including systemic lupus erythematosus (SLE), experimental autoimmune encephalomyelitis (EAE; mouse model of multiple sclerosis), type-1 diabetes (T1D) and rheumatoid arthritis (RA). Rituximab has been shown to deplete both malignant and normal B cells in vivo in animal models as well as patients (Maloney D G et al, Blood. 1994; 84(8):2457-66; Reff M E, et al. Blood. 1994; 83(2):435-45; Schroder C, et al. Transpl Immunol. 2003; 12(1):19-28). It can also deplete normal B-cells from human peripheral blood mononuclear cells (PBMCs) in in vitro experiments (Vugmeyster Y, et al, Cytometry A. 2003; 52(2):101-9; Vugmeyster Y and Howell K. Int Immunopharmacol. 2004; 4(8):1117-24).
Campath-1H (alumtuzumab), an anti-CD52 chimeric IgG1, binds to the CD52 antigen, which is highly expressed on all lymphocytes (Ginaldi L, et al, Leuk Res. 1998 February; 22(2):185-91; Hale G, et al, Tissue Antigens. 1990 March; 35(3):118-27). It is used in patients to deplete malignant lymphocytes and is approved for treating chronic lymphocytic leukemia. It has also shown efficacy in treating multiple sclerosis and is currently in Phase III clinical testing (N Engl J Med 2008; 359:1786-1801; ClinicalTrials.gov NCT00530348 & NCT00548405). It has been shown to deplete normal lymphocytes in vitro as well (Hale G, et al. Blood. 1983 October; 62(4):873-82; Waldmann H and Hale G Philos Trans R Soc Lond B Biol Sci. 2005 Sep. 29; 360(1461):1707-11).
CD37-binding agents are also being tested as potential therapeutics for B-cell malignancies. Emergent Biosolutions (formerly Trubion Pharmaceuticals) developed the CD37-binding agents SMIP-016 and TRU-016 (Zhao et al., 2007, Blood, 110:2569-2577). SMIP-016 is a single chain polypeptide that includes variable regions from a hybridoma and engineered human constant regions. TRU-016 is a humanized version of the anti-CD37 SMIP protein. See e.g. U.S. Published Application No. 2007/0059306. TRU-016 is being tested clinically for the treatment of chronic lymphocytic leukemia (CLL). Boehringer Ingelheim has also disclosed a CD37 binding agent in International Published Application No. WO 2009/019312. However, no CDC activity has been described for any of these binding agents and no in vitro pro-apoptotic activity has been described in the absence of cross-linking agents.
Radio-immunotherapy (RIT) has been attempted using a radio-labeled anti-CD37 antibody MB-1 in two separate trials. Therapeutic doses of 131I-MB-1 were administered to six relapsed NHL patients (Press et al. 1989 J Clin Oncol. 7(8):1027-38; Press at el. 1993, N Engl J Med. 329(17):1219-24). All six patients achieved a complete remission (CR) with a duration of four to thirty-one months. In another trial, 131I-MB-1 was administered to ten relapsed NHL patients (Kaminski et al. 1992 J Clin Oncol. 10(11):1696-711). A total of four patients had a response ranging in duration from two to six months, although only one CR was reported. However, not all patients could be treated due to an unfavorable biodistribution of the radio-label which raised concern for radiation exposure of vital non-target organs. Indeed, RIT related toxicities were observed in these trials including severe myelosupression and cardiopulmonary toxicity. While these clinical data suggest that anti-CD37 radio-immunoconjugates may be effective, these therapies are cumbersome to administer, and at relapse post-RIT patients cannot be retreated with RIT due to the risks associated with high doses of radiation.
To overcome the limitations of RIT, antibody-cytotoxic agent conjugates (ACC), also called antibody-drug conjugates (ADC), have been developed. These are immunoconjugates that include a cytotoxic agent covalently linked to an antibody through a chemical linker which can allow for specific delivery of cytotoxic drugs to cells expressing a protein recognized by the antibody. However, proteins that are poorly internalized are not considered to be favorable targets for such therapeutics. CD37 is structurally similar to CD20 as both antigens contain four transmembrane domains, although CD20 is not part of the tetraspanin family (Tedder et al. 1989, J. Immun. 142: 2560-2568). Antibodies against several B-cell antigens including CD37 and CD20 have been studied for their ability to undergo endocytosis and degradation (Press et al. 1989, Cancer Res. 49(17):4906-12, and Press et al. 1994, Blood. 83(5):1390-7). The anti-CD37 antibody MB-1 was retained on the cell surface and internalized slowly in Daudi lymphoma cells in vitro. The MB-1 antibody also had a low rate of endocytosis and intracellular metabolism in NHL patient cells in vitro. Similar results were obtained with the anti-CD20 antibody 1F5, which was also retained mainly on the lymphoma cell surface and internalized poorly. ADCs of CD20 antibodies have been studied previously but have not demonstrated significantly strong potency, especially when non-disulfide or acid stable linkers are used (see for example Polson et al., 2009, Cancer Res., 69(6):2358-2364). In light of these observations, CD37 has not been considered a favorable target for antibody-drug conjugates.
While their role in cancer treatment has been studied, the potential effect of CD37-directed therapies such as antibodies, antibody derivatives or radio-immunoconjugates on cells involved in autoimmune diseases, inflammatory diseases or other disorders of the immune system is not well understood. Furthermore, none of the compounds described above have been demonstrated to induce depletion of target cells involved in manifestation or progression of these types of diseases.
Therefore, there exists a need for CD37 binding agents including antibodies, antigen-binding fragments thereof, and antibody-drug conjugates (immunoconjugates) as a means to treat autoimmune diseases, inflammatory diseases, or other disorders of the immune system. The present invention addresses that need.